Process and device for treating flue gases

ABSTRACT

A process and a device for treating a carbon dioxide-containing flue gas stream are described. At least a part of the carbon dioxide present is removed from the flue gas stream by means of a scrubber, forming a low carbon dioxide gas stream and a high carbon dioxide gas stream. The high carbon dioxide and water-saturated gas stream formed from the flue gas stream after removal of the carbon dioxide is cooled by means of a refrigeration system  4  in a heat exchanger  3  below the dew point temperature of the water contained in the gas stream and dried by condensing and separating out the water in a condensate separator  5 . The dried gas stream meets the requirements of carbon dioxide products intended for further use.

The invention relates to a process for treating a carbon dioxide-containing flue gas stream, wherein at least a part of the carbon dioxide present is removed from the flue gas stream by means of a scrubber, forming a low carbon dioxide gas stream and a high carbon dioxide gas stream, and also to a device for carrying out the process.

In the utilization of fossil fuels, e.g. coal, mineral oil or natural gas, in power stations or other industrial firing plants, carbon dioxide-containing gas streams, customarily termed flue gases, arise, which are generally released to the environment. Since carbon dioxide is what is known as a greenhouse gas, to which, owing to its climatically harmful action, a substantial adverse effect on the global climate is assigned, technical solutions are required for limiting the carbon dioxide emission to the environment.

Recently, novel power station designs have been proposed in which the carbon dioxide (CO₂) contained in the flue gas is scrubbed out of the flue gas in a scrubbing stage designed, e.g., as an absorption column, downstream of the power station. The power station in this process need not be converted to oxygen combustion, as in what are termed “oxyfuel power stations”, but can be operated conventionally with air combustion. The purpose of these novel designs is to inject the carbon dioxide formed in the combustion of fossil fuels and present in the flue gas into suitable geological formations, in particular into certain rock layers or saltwater-bearing layers, and thereby to restrict the carbon dioxide emission to the atmosphere. The climatically harmful action of greenhouse gases such as carbon dioxide should be reduced thereby. This technology is termed in the specialist field “post combustion carbon capture technology (PCC)”.

Carbon dioxide-containing flue gas streams also arise in other industrial firing plants which are operated with fossil fuels. These include, e.g., industrial furnaces, steam kettles and similar large thermal plants for generating power and/or heat. It is conceivable that also in such plants, the carbon dioxide is separated off from the flue gas streams by means of a scrubber and fed to utilization or storage (e.g. by injection underground).

When carbon dioxide is separated off from flue gases by scrubbing out by means of chemical and/or physical scrubbing media, a high carbon dioxide gas stream arises which is present in the water-saturated state. After a precompression, this gas stream must be dried in order to correspond to the water contents required for high carbon dioxide product streams. If the high carbon dioxide gas stream is intended to be fed, for example, via pipelines to a later use, then for prevention of corrosion and hydrate formation, generally it is necessary to comply with a water content of less than 500 ppmv after a final compression.

The most customary prior art methods to date for reducing the water content of high carbon dioxide gas streams downstream of a flue gas scrubber are drying by means of a molecular sieve absorber station or a glycol scrubber. However, both known processes have specific disadvantages:

Elimination of the water by means of a molecular sieve adsorber station or a glycol scrubber demands a high expenditure in terms of apparatus.

In addition to the adsorbers of the molecular sieve adsorber station, it is necessary to install a multiplicity of control valves (high expenditure on piping). The control of the adsorber station is extensive. For providing the regeneration, additional thermal energy (heating the CO₂ regeneration stream by means of steam or an electric heater) and electrical energy (repeated compression of the expanded CO₂ regeneration stream) are required.

The structure of a glycol scrubber contains an absorption column, a regeneration column, a series of heat exchangers, filters and pumps. In the case of the glycol scrubber, also, additional thermal energy is required for the regeneration.

It is therefore an object of the present invention to configure a process of the type mentioned at the outset and also a device for carrying out the process in such a manner that the drying of the high carbon dioxide and water-saturated gas stream is made possible in an economical manner without the described disadvantages of the previous methods.

This object is achieved according to the invention in terms of the process in that the high carbon dioxide and water-saturated gas stream formed from the flue gas stream after removal of the carbon dioxide is cooled by means of a refrigeration system below the dew point temperature of the water contained in the gas stream and dried by condensing and separating out the water.

In this process the possibility of the formation of gas hydrates (dependent on pressure, temperature and composition) must be taken into account, which formation can be avoided by feeding in glycols, preferably a mixture of monoethylene glycol-water.

In order to separate off water from the high carbon dioxide gas stream, the gas stream must be cooled below the dew point temperature of the water (depending on the pressure of the water-saturated gas). If the temperature falls below the dew point, the water condenses out and can be separated off from the gas stream.

The gas stream including the glycol-water mixture possibly sprayed in should be able to be cooled for this, for example to a temperature of below 10° C. at a pressure of >30 bar. In central European regions this cannot be achieved by cooling water (usual temperatures around 25° C.) In order to provide a cooling medium having the required inlet temperatures, therefore, according to the invention the use of a refrigeration system, in particular a compression or absorption refrigeration system, is proposed.

In contrast to a compression refrigeration system, the absorption refrigeration system has a markedly lower electrical energy consumption, since instead of a compressor a pump can be used. Therefore, according to a preferred configuration of the invention, an absorption refrigeration system is used downstream of the scrubber for drying the high carbon dioxide and water-saturated gas stream.

In a particularly advantageous embodiment of the invention, the absorption refrigeration system is driven by means of thermal energy, in particular in the form of steam and/or remote heat and/or direct firing.

In the case of treatment of a flue gas stream from a coal-fired combustion and/or gasification plant, the thermal energy is expediently obtained from vapours of a coal drying stage connected upstream of the combustion and/or gasification plant or from vapour condensate (if the vapours have already been utilized in advance in another process). Vapours or vapour condensate from coal drying are available as waste heat at a utilizable temperature level (>100° C.) By utilizing this waste heat for driving the absorption refrigeration system, electrical energy is only further required for the circulation pump of the solvent of the absorption refrigeration system. Considerable savings of electrical energy are achieved thereby.

To comply with the desired product requirements for a CO₂ stream intended for further use or storage, the high carbon dioxide gas stream is dried preferably to a water content of less than 500 ppmv.

In this case the high carbon dioxide gas stream is expediently dried at a pressure between 30 and 60 bara, preferably between 35 and 40 bara.

The present invention is primarily intended for processing flue gases from conventional combustion plants. The carbon dioxide-containing flue gas stream is formed in this case in an industrial firing plant in which fossil fuels are burnt with combustion air. This flue gas stream is preferably subjected to a scrubbing in an absorption column, with subsequent scrubbing medium regeneration, for separating off carbon dioxide from the flue gas stream. By expelling gaseous components during the scrubbing medium regeneration, the high carbon dioxide gas stream is expediently formed, whereas the low carbon dioxide gas stream is taken off from the absorption column.

Preferably, the carbon dioxide is removed from the carbon dioxide-containing flue gas stream by means of a scrubber having a physically and/or chemically acting scrubbing medium. In this process the scrubbing medium expediently contains as a component at least one amine.

In a particularly preferred configuration of the invention, accordingly, the high carbon dioxide gas stream is formed in a regeneration of a scrubbing medium used for scrubbing, which scrubbing medium contains as a component at least one amine.

The carbon dioxide removed from the flue gas stream can finally be fed to a utilization or storage, in particular injection underground, whereas the low carbon dioxide gas stream can be released to the atmosphere with a considerably reduced climatically harmful action.

The invention further relates to a device for treating a carbon dioxide-containing flue gas stream having a scrubbing appliance for scrubbing out from the flue gas stream at least a part of the carbon dioxide present, forming a low carbon dioxide gas stream and a high carbon dioxide gas stream.

The object in question is achieved in terms of the device in that a refrigeration system for cooling and drying the high carbon dioxide gas stream is connected downstream of the scrubbing appliance. In this case, in addition, an appliance for spraying in glycols, in particular a glycol-water mixture, can also be provided.

A particularly preferred configuration of the device envisages that the refrigeration system is constructed as an absorption refrigeration system.

The invention has a great number of advantages:

Refrigeration systems require considerably lower capital and operating costs compared with the molecular sieve adsorber stations or glycol scrubbers customarily used for drying.

The possibility of utilizing thermal energy (advantageously waste heat) for generating refrigeration in the absorption refrigeration system is particularly advantageous. In particular when vapours from an upstream coal drying stage are used in coal-fired combustion or gasification plants, the energy requirement for drying can be considerably reduced. Instead of vapours, vapour condensate can alternatively be used, if the vapours have already been used in another process.

The simple and robust construction is a further advantage of the use of an absorption refrigeration system for drying the high carbon dioxide and water-saturated gas stream.

The invention is suitable for all conceivable industrial firing plants in which carbon dioxide-containing gas streams arise. These include, e.g., fossil-fuelled power stations, industrial furnaces, steam kettles and similar large thermal plants for power and/or heat generation. Particularly advantageously, the invention can be used in industrial firing plants which are supplied with air as combustion gas. In particular, the invention is suitable for coal power stations in which the CO₂ is scrubbed out of the flue gas and injected below ground (“CCS—carbon capture and storage”).

The invention will be described in more detail hereinafter with reference to an exemplary embodiment illustrated diagrammatically in the figure:

The figure shows a block diagram of a processing plant for a high carbon dioxide and water-saturated gas stream from a carbon dioxide scrubber of a coal power station.

In a scrubber which is not shown in the figure, a high carbon dioxide gas stream is separated off from a carbon dioxide-containing flue gas of a coal-fired power station, which high carbon dioxide gas stream is intended for storage below ground. The high carbon dioxide gas stream is precompressed by means of a compressor 1 and then passed through a heat exchanger 2 where the gas stream is precooled by means of cooling water KW. A glycol-water mixture is then sprayed in, wherein the majority of the glycol originates from the glycol processor 7 (concentration of the glycol-water mixture separated off in 5). In a downstream heat exchanger 3 further cooling by means of the absorption refrigeration system 4 then proceeds. For supplying the absorption refrigeration system with thermal energy, vapours BD and vapour condensate BK from an upstream coal drying stage are used. The glycol-water mixture condensed out in the heat exchanger 3 as a consequence of the cooling below the dew point temperature of the water is separated off in a downstream condensate separator 5. The gas stream dried to a water content of less than 500 ppmv is finally compressed in a final compressor 6 to the pressure intended for the further use of the gas stream (e.g. for injection below ground). 

1. Process for treating a carbon dioxide-containing flue gas stream, wherein at least a part of the carbon dioxide present is removed from the flue gas stream by means of a scrubber, forming a low carbon dioxide gas stream and a high carbon dioxide gas stream, characterized in that the high carbon dioxide and water-saturated gas stream formed from the flue gas stream after removal of the carbon dioxide is cooled by means of a refrigeration system (4) below the dew point temperature of the water contained in the gas stream and dried by condensing and separating out the water.
 2. Process according to claim 1, characterized in that, as a refrigeration system (4), an absorption refrigeration system is used.
 3. Process according to claim 2, characterized in that the absorption refrigeration system is driven by means of thermal energy, in particular in the form of steam and/or remote heat and/or direct firing.
 4. Process according to claim 3, characterized in that, in the case of treatment of a flue gas stream from a coal-fired combustion and/or gasification plant, the thermal energy is obtained from vapours (BD) and/or vapour condensate of a coal drying stage connected upstream of the combustion and/or gasification plant.
 5. Process according to claim 1, characterized in that the high carbon dioxide gas stream is dried to a water content of less than 500 ppmv, wherein a glycol-water mixture is sprayed in for avoiding gas hydrate formation.
 6. Process according to claim 1, characterized in that the high carbon dioxide gas stream is dried at a pressure between 30 and 60 bara, preferably between 35 and 40 bara.
 7. Process according to claim 1, characterized in that the high carbon dioxide gas stream is formed in a regeneration of a scrubbing medium used for scrubbing, which scrubbing medium contains as a component at least one amine.
 8. Process according to claim 1, characterized in that the dried, high carbon dioxide gas stream is fed to a utilization or storage.
 9. Device for treating a carbon dioxide-containing flue gas stream having a scrubbing appliance for scrubbing out from the flue gas stream at least a part of the carbon dioxide present, forming a low carbon dioxide gas stream and a high carbon dioxide gas stream, characterized in that a refrigeration system (4) for cooling and drying the high carbon dioxide gas stream is connected downstream of the scrubbing appliance.
 10. Device according to claim 9, characterized in that the refrigeration system (4) is constructed as an absorption refrigeration system. 